When an armoured truck is robbed by three masked robbers packing a hefty arsenal, all clues point to Ted Gaynor - a former marine played by Farscape's Ben Browder in another of this show's nods to the geek viewership who also just happens to be Diggle's old commanding officer.

Dig (David Ramsey) won't believe that his former mentor is the one behind the violent heist, but ultimately his words don't make much of an impact on Oliver Queen (Stephen Amell) who immediately hunts down and attacks Gaynor - only to find his bodyguard and partner siding with his military pal...

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Oliver and Diggle clash, first physically and later verbally - Dig mocks our hero's unflinching belief in Robert Queen's list, leading Ollie to snap back, "I trust my father!" - the same father who lied to you for years, concealing his true motives and activities from your entire family? Sure, makes sense.

Diggle is this writer's favourite Arrow character - when Oliver acts like a hot-head, which he frequently does, he's the cooler head that prevails, so to have a Dig-centric episode is most welcome. In 'Trust But Verify', we get to touch a little on his past, his personal life and his romantic feelings for his ex-sister-in-law Carly (Christie Laing).

But Diggle is the one who eventually uncovers the wholly unsurprising truth - he's the only one surprised when it transpires that Gaynor is heading up the corrupt and thieving Blackhawks.

When the crook threatens Carly's life, Dig is coerced into helping his former friend pull off a new heist, but the Blackhawks are an incompetent bunch of villains, handing their captive a grenade launcher which he wisely turns on them the first chance he gets.

Gaynor ends up with a crossbow bolt through the chest courtesy of Oliver, who ultimately admits that he should have placed his trust in Diggle and not his late father's list. Ollie may act like a dolt sometimes, but he's growing little by little.

Elsewhere this week, Malcolm Merlyn (John Barrowman) invites his son Tommy (Colin Donnell) and girlfriend - if that's not too much of a label for her - Laurel (Katie Cassidy) to dinner to mend a few fences. But it's all just a ruse to shut down Tommy's late mother's free clinic, which means more tough times for the younger Merlyn - Tommy is this show's whipping boy and the poor lad really doesn't deserve it.

A tearful Tommy is down in the dumps and - shocker - Laurel is actually pretty nice to him, but little does he know that the death of his mother had a profound effect on widower Malcolm, who became the Dark Archer as a result - he's a sort of Batman gone wrong then.

Their frequent meetings lead to a slightly tiresome sub-plot in which Thea (Willa Holland) falsely suspects that Malcolm and Moira are having an affair - but this does at least raise Ollie's suspicions that his mother may not be a perfect saint. And it's possible that Oliver's doubts about his father's own fidelity might lead him to further question the broader quest that Robert sent him on, instead of blindly following that path.

In general though, while Willa Holland's great, Arrow seems to be struggling to find enough for her to do, relegating Thea to inconsequential sub-plots and generic 'moody teen' behaviour. Hopefully the imminent introduction of Colton Haynes as her love interest - and potential Hood sidekick - Roy Harper will help counter this problem.

Rounding off 'Trust But Verify' are a few sparse island flashback sequences - past Oliver starts showing some signs of self-reliance, posing as a member of Edward Fyers's paramilitary force in order to track down his mentor Yao Fei (Byron Mann), who was last seen being spirited away by Deathstroke...

But Ollie's unmasked relatively quickly - he's still not quite the chiselled, salmon-ladder-scaling tough guy he will later become. Held hostage, Ollie is shocked to learn that Yao Fei is actually employed by Fyers and this came as a genuine surprise to us too - chiefly because it's a twist that doesn't appear to make much sense at this point.

Why would Fei train Ollie in the ways of bad-ass archery if he was working for the opposition? Hopefully next week we'll be dealt an explanation that passes muster.

'Trust But Verify' is a little light on action - usually one of this show's greatest strengths - but the heavy focus on Diggle, a strong guest turn from Ben Browder and intriguing hints at what's to come on the Dark Archer / Vertigo front mean that this Arrow instalment remains 42 minutes of solid entertainment.